<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .parent{
            width: 500px;
        }
        .f1{
            float: left;
            background-color: green;
            width: 100px;
            height: 100px;
        }
        .main{
            height: 100px;
            background-color: pink;
        }
    </style>
</head>
<body>
    <div class="parent">
        <div class="f1"></div>
        <div class="main">121212121212112112121121211121212121</div>
    </div>
</body>
</html>